First, let's define "tonkotsu". It translates into "pig bone" or "pork bone" in Japanese. But none of us are going to order a bowl of "pig bone ramen". So tonkotsu generally refers to the thick, rich broth created by boiling the pork bones. Simply put - it's the Holy Grail of ramen broths.
You may never notice the silky texture of the tonkotsu broth as part of the finished bowl of ramen. By the time it reaches your restaurant table, the broth is filled with flavors, spice, noodles and toppings. At that point, it's often about "taste" instead of texture. But if you are going to make a tonkotsu broth at home, you can easily examine how silky smooth that broth base gets after you finish the boiling process.
Now, back to our central question: why do you boil tonkotsu broth for so long? Simply put, you are extracting marrow and collagen from the pork bones. You'll never get a truly gelatinous tonkotsu broth with a slow simmering approach as we do with clear and aromatic broths. Nor will you get the best version of that tonkotsu broth with a short boil. A couple of hours just won't cut it.
We boil our tonkotsu broth for 12 or more hours. Often we finish at more than 13 hours. We use pork feet bones along with pork neck and occasionally pork back bones. As the boiling starts, the bones start to break down or collapse from the high heat and boiling water. As hours pass, the collagen, marrow and fat from the bones are emulsified into the water.
In the ramen making classes I teach, students are always surprised by the color of the broth after the extended boiling. We use a very pure approach: bones + water + boil. The finished liquid tonkotsu broth is quite milky white in color. That's because the marrow, collagen and all the goodness of the bones are dissolved and woven into the liquid broth over the many hours of boiling.
We use ramen meters to test the viscosity of the broth after 12 hours of boiling. Digital meters are expensive. Analog versions are less so. Still, I always tell my students that if you are just doing this at home and don't want to invest the money in ramen meters, make your first boil at least 8 hours. The next time boil for 10 hours to notice the difference. Then on your third attempt, boil for 12 hours. This way you can see how the marrow and collagen breakdown change the broth in the last 4 to 5 hours of boiling.